For economic and reliability considerations electrical energy generating plants in geographically large areas are being organized into single power pools. It is essential that power be transmitted efficiently and, to this end, the transmission voltages have been increasing over recent years so that a potential of the order of 345-KV is not uncommon and consideration is being given to 1,000-KV and even higher potential lines. Such lines, because of ecological, reliability and physical site problems, are preferably buried underground and cross rivers underwater.
The ultrahigh-voltage lines hum and crackle even louder than do the conventional transmission circuits. Noise levels of up to 60 decibels, comparable to city traffic, have been recorded under them.
The electric fields around the lines are strong enough so that if a person underneath one touches a sizable metal surface, such as the side of a vehicle, he can receive a pronounced shock. Engineers say there is no way of insulating overhead wires to prevent such leakage.
The lines carry alternating current, oscillating at the standard rate of 60 cycles a second. It is these fluctuations, combined with the strength of the current, that are thought to have harmful physiological effects.
Accordingly, there is a trend to install such lines underground in pipes. In a typical 345-KV gas-insulated system, a 6-in.-diameter conductor is centered (by spacers) inside an 18-in.-diameter aluminum tube. The gas, usually SF.sub.6 under moderate pressure, is pumped in to fill the empty space. It fully insulates the conductor from the outer tube, so that the tubes can be put side by side. The commonly used high tension conductors of bare metal would have to be 10 feet apart.
The present invention contemplates an outer conduit which will contain the transmission line, protect it, provide electrostatic shielding, permit the inclusion of cooling oil or gas and be corrosion resistant.